mainly to avoid the pattern of
* get username/hostname/rolename
* optionally find the corresponding ACL_USER and ACL_ROLE
* allocate memory, concatenate username/hostname/rolename
* call a function passing only this memory as an argument
** use concatenated username/etc to find ACL_USER and ACL_ROLE again
** do something
* free the object

Also to undo push_dynamic we use pop_dynamic now,
not a linear search/scan through the dynamic array.

as a bonus, role@ is now an invalid way to refer to a role.